db={"chatbots":[{name:"First",campaigns:[{name:"Campaign #1",channels:[{_id:"eyRyZ1gD0",name:"Channel #1",budget:12},{_id:"tsKH7WxE",name:"Channel #2",budget:14}]},{name:"Campaign #1-2",channels:[{_id:"eyRyZ1gD0",name:"Channel #1",budget:1},{_id:"tsKH7WxE",name:"Channel #2",budget:1},{_id:"abc",name:"Channel #3",budget:1}]}]},{name:"Second",campaigns:[{name:"Campaign #2",channels:[{_id:"eyRyZ1gD01",name:"Channel #4",budget:121},{_id:"tsKH7WxE1",name:"Channel #5",budget:141}]}]}],"chatbotusers":[{name:"One",campaign_channel:"eyRyZ1gD0"},{name:"Two",campaign_channel:"eyRyZ1gD0"},{name:"Three",campaign_channel:"eyRyZ1gD0"},{name:"Four",campaign_channel:"eyRyZ1gD0"},{name:"Five",campaign_channel:"eyRyZ1gD01"},{name:"Six",campaign_channel:"tsKH7WxE1"},{name:"Seven",campaign_channel:"tsKH7WxE"},{name:"Eight",campaign_channel:"tsKH7WxE"}]}

db.chatbots.aggregate([{$unwind:"$campaigns"},{$unwind:"$campaigns.channels"},{$lookup:{from:"chatbotusers",localField:"campaigns.channels._id",foreignField:"campaign_channel",as:"users"}},{$addFields:{"channels":"$campaigns.channels",campaigns:"$campaigns.name"}},{$addFields:{"channels.users":{$size:"$users"}}},{$project:{users:0}}])